Reflex sights work on the principle that an aiming mark, usually a dot, is imaged at infinity by a concave mirror. Owing to the fact that this concave mirror is constructed as a semitransparent mirror, it simultaneously opens up the view to the target. The target and aiming mark must be acquired without accommodation of the eye through an appropriate design of the general system.
Typically, these devices are virtually non-magnifying aiming devices which are outfitted with a virtual luminous mark imaged at infinity. Therefore, the names red dot sight or collimator sight are also commonly used for these devices.
A substantial advantage of reflex sights over telescopic sights consists in that there is no system-dependent predetermined distance between the sight and the eye for their use. This distance can be a few centimeters but can also be greater than a meter without affecting function. Since there is only a slight magnification, if any, sighting can also be carried out with both eyes open. This allows for good observation of the target field.
In most of these devices, the luminous dot, which is preferably red, is generated by a light emitting diode. The size of the light emitting diode and the focal length of the collimating optics determine the apparent size of the luminous dot. To obtain a luminous dot with only a small target coverage, either a long focal length or a very small dot size must be selected. A long focal length means a correspondingly large device.
A compact construction requires that the focal length is relatively small and that the aperture is comparatively large. The focal length for these compact sighting devices is in the range of 25 mm.
Therefore, the diameter of the luminous dot for a small dot size for the class of compact sights under discussion is 50 μm or less.
With respect to the collimating optics used for this purpose, it must be ensured that the view is not optically modified on the one hand and that the luminous dot is imaged at infinity on the other hand. This is achieved by means of a partially reflective layer on the concave lens surface facing the observer, while the other side of the lens is adapted in such a way that the optics have no refractive power.
There are various embodiment forms of this optical system for achieving a high-quality imaging of the dot and an unaltered viewing image. In the simplest instance, a tilted lens is used. However, it is usually necessary to use off-axis segments. Also, the optical imaging quality can be improved even further by aspherical surfaces. Such aspherical surfaces are formed, for example, as thin plastic layers with a glass lens as supporting medium. These are known as replica optics. Further, there are systems which are composed of a plurality of lenses or which contain cover plates in addition.
In terms of the construction of the optics, there is either a tubular constructional shape with lenses or cover plates at each end or an open constructional shape with a freestanding lens or lens group. The open construction has proven successful for compact sights in particular. It offers a large field of view because the optics are enclosed only by a narrow rim, and there is no tube. These devices are usually not watertight, but rather water-resistant or spray-resistant. On the other hand, the tube type construction is more robust, substantially larger and, therefore, also heavier. Another disadvantage of this constructional type is the limited visual angle. They can also have a slight magnification so that the visual angle is further reduced. The focus of application is military, where robustness and tightness are particularly important. Sights in a tubular constructional form are disclosed, for example, in European Patent EP 1 182 419 B1 and U.S. Pat. Nos. 5,189,555 A and 5,440,387 A, in which the aiming mark is arranged inside two delimiting optical component parts.
An important quality feature of reflex sights is the parallax compensation of the aiming mark. Absence of parallax means that the image of the aiming mark and the targeted object lie in a plane. This prevents the aiming mark from moving in front of the object when the sighting device is looked through off-center.
In order to achieve minimal parallax errors, the higher-quality sighting devices are factory set at a parallax-free observation distance of, e.g., 40 m or 100 m.
Devices of this type have an elevation adjustment and lateral adjustment to bring the position of the sighting dot and the impact point into alignment with one another. This adjustment should function in two orthogonal axes with as little play as possible and must not affect the parallax adjustment. Apart from the mechanical precision, the stability of the parallax compensation over the entire adjustment range is also critically influenced by the flattening of the image field of the collimating optics.
Sighting devices in which luminous aiming marks are reflected in are typically constructed from a convex-concave lens, the concave surface being reflective and facing the eye. The lens is constructed in such a way that it has no, or only a slight, optical power with ray passage in a straight line. Also, only one lens section is typically used so that a corresponding centric arrangement of the aiming mark is possible without obstructing the image field and sighting window. A representative arrangement is described in U.S. Pat. No. 4,346,995.
There are also other differences in these sighting devices with respect to the alignment of the optical axis of these systems in relation to the viewing direction. In sighting devices in which the optical system is tilted relative to the observation direction, the wedge effect causes a ray deflection which allows the object to appear in a different direction. As is shown in U.S. Pat. No. 5,594,584 A, the effort to eliminate this ray deflection leads to two-part lens systems in which the lenses are displaced relative to one another. However, de-centered systems of this kind have substantial flaws in imaging quality from the start.